Patterned pavement markings with upright retroreflectors

ABSTRACT

An improved pavement marking sheet material, and a method for making such pavement marking sheet material. The pavement marking sheet material comprises a selected configuration of upright retroreflectors. The upright retroreflectors are arranged in a selected configuration so as to maximize reflectivity as measured at actual observation geometries by allowing light to be incident to effective portions of the retroreflective elements on the upright retroreflectors.

This is a continuation of application Ser. No. 08/247,050 filed May 20,1994 now abandoned.

FIELD OF THE INVENTION

The present invention relates to an improved pavement marking material,and a method for making such pavement marking material. In particularthe invention relates to a pavement marking material with a selectedconfiguration of upright retroreflectors, such configuration selectedfor maximum retroreflectivity when measured at geometries correspondingto actual observation conditions as from a typical automobile. Theinvention also relates to patterned pavement markings with specularlyreflective pigments such as pearlescent pigments and aluminum flakes.The invention also relates to patterned pavement markings withultraviolet-fluorescent pigments.

BACKGROUND

Pavement markings, such as those delineating the centerline and edgelineof a roadway are important in order to provide visual guidance for motorvehicle drivers. An ongoing challenge to the industry is to providepavement markings of increasing brightness, particularly at nighttimewhen visibility is limited.

Currently, pavement markings typically rely on an optical systemcomprising transparent microspheres partially embedded in a binder layercontaining diffusely reflective pigment particles such as those oftitanium dioxide (TiO₂) or lead chromate (PbCrO₄) as reflectors. In use,light from the headlamp of a vehicle enters the microsphere and isrefracted to fall on a diffusely reflective pigment. Some portion of thelight is returned generally along the original entrance path in thedirection of the vehicle so as to be visible to the vehicle driver.

Pavement marking tapes with reflective protrusions or protuberances ontheir upper surfaces, sometimes referred to as patterned or profiledpavement markings, are more desirable than flat pavement markings toimprove visibility, especially at night and when the roadway is wet. Theraised pattern facilitates runoff of rain water. Also, the raisedpattern provides non-horizontal surfaces to support retroreflectivemicrospheres. The non-horizontal surface isolates the optical surfacesof the microspheres from abrasive wear by traffic and provides a moreeffective orientation of the microspheres and pigments. A raised patternalso permits use of a highly efficient combination of microspheres andspecular reflective pigments.

U.S. Pat. No. 4,388,359 (Ethen et al.) discloses a pavement markingsheet having protuberances on its upper surface which carry partiallyexposed microspheres by which the protuberances are made retroreflectiveand which are separated by recessed areas in which microspheres arefully embedded and not exposed, so as to improve the daytime appearanceof the sheet material.

U.S. Pat. Nos. 4,988,555 (Hedblom) and 4,988,541 (Hedblom) disclose apavement marking comprising a base sheet and integral protrusions havinga top and side surfaces. Selected side surfaces are covered with a beadbond layer, into which retroreflective microspheres are subsequentlypartially embedded. U.S. Pat. No. 5,227,221 discloses a pavement markingcomprising a base sheet and integral protrusions having a top and sidesurfaces, where all side surfaces and the top surface of each protrusionare covered with a bead bond layer.

All the above-mentioned patents disclose pavement markings embodying auniform pattern of protrusions; that is, the size, shape and spacing ofthe protrusions are substantially the same across the face of themarking.

U.S. Pat. No. 3,758,192 (Bingham) discloses the use of specular pigmentssuch as nacreous pigments or aluminum flakes dispersed in a binder. Thiscoating is applied to fabrics, and transparent retroreflectivemicrospheres applied to the coating to make retroreflective clothing.

Retroreflectivity is currently typically measured by an instrument inthe laboratory, at fixed entrance and observation angles, according toASTM 01.26.23. Recent work (Transportation Research Record 1409published 1994 by the Transportation Research Board) has shown that theentrance angle at which light is incident and observation angles fromwhich a driver actually views a pavement marking, referred tohereinafter as "approximate driver geometries", are greatly differentfrom those geometries at which reflectance values are currently measuredin the laboratory, referred to herein as "laboratory geometries".Constructions that yield a lower brightness compared to a standard whenmeasured at laboratory geometries may result in much higher brightnessvalues compared to that standard when measured at approximate drivergeometries.

SUMMARY

The present invention provides a pavement marking material withincreased retroreflective performance due to selected configuration ofupright retroreflectors as described below. The selected configurationresults in minimized shadowing so that the most optically efficientretroreflective elements, those on the vertical sides of the uprightretroreflectors, are exposed to incident light. Shadowing refers to thevertical aspect of an upright retroreflector blocking, or shadowing,nearby upright retroreflectors such that the retroreflectivemicrospheres on the vertical sides of the shadowed uprightretroreflector are not visible and thus are not utilized.

When evaluating retroreflective brightness or retroreflectivities ofvarious constructions at the approximate driver geometries, relativeretroreflectance rankings have been found to have dramatically changedfrom those determined at laboratory geometries. In order to accuratelycompare retroreflectivities, and therefore practical utility of pavementmarkings, it is necessary to not only measure all constructions beingevaluated at a selected range of geometries, but also to ensure that thechosen geometries are approximate driver geometries.

In brief summary, the invention provides a pavement marking materialcomprising a continuous polymeric base sheet having a plurality ofupright retroreflectors on its upper surface. The upper surface is madeup of a plurality of segments arranged in a selected manner, with eachsegment having a front edge, rear edge, and two side edges. Each segmentalso has one upright retroreflector or an array of a plurality ofupright retroreflectors positioned along its front edge and a spacingzone that is free of upright retroreflectors along its rear edge suchthat the upright retroreflectors are positioned closer to the front edgethan to the rear edge. The longitudinal length of the spacing zone(i.e., in the direction of the front edge/rear edge axis) is greaterthan the average distance between adjacent upright retroreflectors. Thepavement marking material comprises a plurality of segments arrangedwith the front edge of a segment abutting the rear edge of an adjacentsegment. The term segment merely refers to the spatial arrangement ofspecified portions of the upper surface. The upper surface may be anintegral sheet with retroreflectors formed therein or may actually bemade up of separate pieces that are tiled or arranged together. Severalexamples of possible segments are shown in FIG. 2. When applied to aroadway, the marking material is oriented such that the front edge torear edge axis of each segment is parallel to the major direction oftravel.

In some embodiments, the array of the segment comprises a plurality ofrows. In such instances, they are preferentially offset from each otherso that the front surfaces of upright reflective elements in each roware exposed to incident light. Selected side surfaces are covered with abead bond layer, into which a plurality of particles are subsequentlypartially embedded. The particles include retroreflective microspheresand skid-resistant particles. The bead bond layer, or binder layer, maycontain reflective pigments. The reflective pigments may be diffuselyreflective pigments such as titanium dioxide or lead chromate.Alternatively, the reflective pigments may be specularly reflectivepigments such as pearlescent (nacreous) pigments or aluminum flakes. Inaddition to the reflective pigments, the binder layer may also includeultraviolet-fluorescent (UV-fluorescent) pigments.

The primary use of this invention is as a patterned or profiled pavementmarking. The invention has greater retroreflectivity at entrance anglescommon to roadway delineation than comparable constructions notemploying an optimized configuration of upright retroreflectors. Theoptimized configuration results in the exposure and utilization of themost optically effective retroreflective elements, those on the verticalsides of the upright retroreflective elements. Patterned or profiledpavement markings with the upright retroreflectors arranged closertogether will have vertical sides of subsequent upright retroreflectorsblocked or shadowed by the upright retroreflector in front of it.Therefore, most of the retroreflective effect that results from such apavement marking is from the retroreflective elements on or near the topsurfaces of the upright retroreflectors, which are less opticallyefficient than those on the vertical sides of the uprightretroreflectors. In short, the major advantage of an uprightretroreflector, the ability to utilize the highly efficient specularlyreflective pigments and reflectors with the microspheres, is lost orgreatly diminished.

Pavement markings of the invention with their optimal configuration ofupright retroreflectors are also expected to have better conformance tothe road than a comparably constructed pavement marking with more ofthese upright retroreflectors spaced closer together. The reason thepavement marking will be more conformant is because it has a greaterpercentage of depressed areas void of upright retroreflectors. Thesedepressed areas are thinner, resulting in better overall conformability.

Pavement markings of the invention may also have improvedskid-resistance since there are larger areas of flat base sheet betweenarrays of upright retroreflectors.

In addition, pavement markings of the invention will be less expensiveto manufacture. Since the expensive microspheres are only applied to theupright retroreflectors, the reduced numbers of upright retroreflectorsresults in less microspheres being needed. Also, the materials whichmake up the base sheet will also be minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained with reference to the drawing,wherein:

FIG. 1 is a schematic diagram of a pavement marking material withupright retroreflectors.

FIG. 2 is a schematic diagram of a pavement marking material similar toFIG. 1 with a selected configuration of upright retroreflectors tomaximize reflectivity by minimizing shadowing.

FIG. 3 is a schematic diagram demonstrating the calculation of optimumheight and spacing of the upright retroreflectors for a given entranceangle.

FIG. 4 is a schematic diagram of an embodiment of a protrusiondemonstrating the optics of the incident light falling on themicrospheres on the sides of the upright retroreflective elements,demonstrating the criticality of the vertical surfaces being exposed toincident light.

FIG. 5 is a schematic diagram of an embodiment of a protrusion withbinder layer and particles over the entire exposed surface.

These figures, which are idealized and not to scale, are intended to bemerely illustrative and non-limiting.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As mentioned above, the invention provides a pavement marking with aselected configuration of upright retroreflectors so as to maximizereflectivity by minimizing shadowing. Spacing the uprightretroreflectors so that the vertical sides of the uprightretroreflectors are exposed ensures that the most optically efficientretroreflective elements, those on the vertical sides of the uprightretroreflectors, will be utilized.

An upright retroreflector may be comprised of either a protrusion whichis an integral part of the base sheet, or a separately formed protrusionwhich is applied to a flat base sheet. In the case of protrusions whichare an integral part of the base sheet, all (see FIG. 5) or selectedsurfaces (see FIG. 4) of the protrusion may be covered with a binderlayer, into which a plurality of particles are subsequently embedded.The particles include retroreflective microspheres or skid-resistantparticles, or a combination thereof. The binder layer may containreflective pigments and UV-fluorescent pigments. In the case ofprotrusions applied to a flat base sheet, the protrusions may be formedfor example by extruding a rod of material, for example a thermoplasticcontaining reflective pigment, in a desired shape and adheringretroreflective microspheres to the rod, which is then cut into wafers.An alternative would be to embed ceramic microspheres into athermoplastic pellet containing a specular reflective pigment. The baseshape of these protrusions, that is, the shape of the base of theprotrusion in the plane parallel to the surface of the base sheet, maybe of any suitable shape, for example a polygon or a circle or an oval.Alternatively, sheets of retroreflective material may be applied to therod before it is cut into wafers. Illustrative examples ofretroreflective materials that may be used in this manner includemicrosphere-based retroreflective sheeting, e.g., encapsulated-lens,exposed-lens, or embedded-lens retroreflective sheeting, and cube cornertype retroreflective sheeting. In view of the relatively small size ofthe upright retroreflectors in this invention as compared to the typicalsize of the cells in conventional cellular products, it will typicallybe preferred to use embedded-lens or exposed-lens microsphere-basedproducts or aluminized cube corner type products rather than cellularmicrosphere-based or cube corner products. Particles may optionally beapplied to the top surface of the wafers if desired. In eitherembodiment of protrusion, whether the protrusion is an integral part ofthe base sheet or is a separately formed protrusion which is thenapplied to a base sheet, the protrusions may be coated with binder layerand microspheres or sheets of retroreflective material in such a manneras to be one color when viewed from one direction and another whenviewed from the other direction. For instance, one kind ofretroreflective material may be placed on one side of the uprightretroreflector and a second material placed on the other side of theupright retroreflector, so that the color and/or retroreflective effectdepends on the direction of approach.

It is generally accepted that for night driving, the driver is lookingon the order of 50 meters ahead of the vehicle most of the time, i.e.,just beyond the reach of the low-beam or dipped headlights on the road.At this distance and beyond only the raised surfaces of a typicalprofiled pavement marking are visible to the driver, as the flat base ofthe marking is obscured by the raised elements. At closer distances,such as 5 to 15 meters, the driver usually observes the lane markings asperipheral images and makes use of them for lateral positioning withinthe lane. At these closer distances, the portion of the flat basebetween the protrusions tend to be the dominant visual aspect of themarking.

Nighttime visibility performance of pavement markings is predicted by asurrogate method of laboratory or field photometric measurement. Thereare currently several photometric systems in use that vary widely ingeometric and precision capability.

With the advent of pavement markings with a variety of retroreflectiveoptical systems and surface characteristics, measurements in thelaboratory and field have been found to lack correlation with themarkings' actual visibility performance as experienced by motor vehicledrivers. (Transportation Research Record 1409 published 1994 by theTransportation Research Board)

For example, as measured by conventionally used photometric systems, aconventional pavement marking with a large number of uniformly arrangedupright retroreflectors will exhibit a higher retroreflectivity than asimilar pavement marking with every second and third row of uprightretroreflectors removed. When measured at approximate driver geometrieshowever, the pavement marking with every second and third row of uprightretroreflectors removed, that is, with 2/3 of its uprightretroreflectors removed, may have a higher reflectivity. This is becausewhen the upright retroreflectors are closer together, the sides of thesuccessive upright retroreflectors are covered or shadowed by thepreceding upright retroreflector, so that only the retroreflectiveelements on or near the tops of the upright retroreflectors are struckby incident light. The most optically efficient microspheres, those onthe vertical sides of the upright retroreflectors, are not utilized tothe extent that the vertical surfaces are covered. The reason theretroreflective surfaces on the vertical sides of the uprightretroreflectors are more efficient is because incident light from theheadlamp of a motor vehicle will fall closer to normal on these verticalretroreflective surfaces than it will on retroreflective surfaces on theflat portion of the pavement marking. The closer to normal the lightfalls on the retroreflective surface, the more efficient theretroreflective surface becomes because of its increased ability toallow more light to enter the retroreflective element.

The entrance angle may be defined as the angle between the illuminationaxis and the retroreflector axis; that is, the angle between the lightsource and an angle perpendicular to the surface being viewed, in thiscase the surface of a pavement marking, e.g., the portion from which anumber of microspheres partially protrude. The observation angle may bedefined as the angle between the illumination axis and the observationaxis.

The instruments available to measure reflectivity in the laboratory orin the field are generally capable of measuring entrance angles of about86.0° to 86.5°, and observation angles of about 1.0° to 1.5°. ASTM01.26.23 standard calls for a geometry of 86.0°/0.2°, i.e., 86.0°entrance angle and 0.2° observation angle. These are unrealisticgeometries for pavement markings, as these geometries would in effectplace the viewer some distance behind the vehicle from which the objectis being viewed. Following are some calculated geometries for anAmerican mid-size automobile at selected distances between a lightsource and a pavement marking; 30 meters is approximately 88.5°/1.0°, 50meters is approximately 89.3°/0.6°, and 80 meters is approximately89.6°/0.4°. As can be seen from the selected distance examples, theentrance angles encountered by automobile drivers are much higher thanthe 86.0° specified in the ASTM standard.

A general formula which may be used to calculate a desired distancebetween protrusions of uniform height in order to completely expose(i.e., free of occlusion at actual presentation orientations) thevertical surfaces of subsequent protrusions and thereby maximizeretroreflectivity as measured at approximate driver geometries is givenby

    distance=height of protrusion/tan(90°-entrance angle)

The distance calculated from this formula will be that distance whichwill result in no shadowing; that is, the entire vertical surface ofeach protrusion will be fully exposed to incident light at that entranceangle. It should be kept in mind that optimal spacing of protrusions interms of other aspects such as water drainage, wear resistance, or skidresistance, may be different from the spacing of protrusions calculatedfor optimal retroreflective performance. If the distance betweenprotrusions is sufficiently large relative to the width of the base ofthe protrusions, the width of the protrusions will have minimal impacton shadowing. As the protrusions are spaced closer together however theimpact of the protrusion width increases.

The material of the pavement marking should be of sufficient strength toresist permanent deformation or excessive wear under the weight oftraffic. The base sheet is typically a resilient polymeric base sheet.If the base sheet comprises integral protrusions, the protrusions have atop surface and at least one side surface. The tops of the protrusionsessentially define a plane substantially parallel to the surface of thesheet. Preferably, less than 45 percent of the area of the base sheet iscovered by protrusions. More preferably, less than 15 percent of thearea of the base sheet is covered by protrusions.

A suitable base sheet may preferably be formed using known methods andmaterials, such as described in U.S. Pat. No. 4,490,432 (Jordan). Thebase sheet described in the patent comprises elastomer precursors, notyet vulcanized or cured, which therefore permit viscoelasticdeformation. Exemplary materials are acrylonitrile-butadiene polymers,millable urethane polymers and neoprenes. Extender resins may beincluded. Particulate fine-diameter fillers, such as silica, may beincluded. Pigments, such as titanium dioxide, are preferred in the basesheet to provide a white diffuse surface to uncoated portions of thebase and protrusions. Another useful pigment is lead chromate whichimparts a yellow color.

The binder layer may contain reflective pigments. The pigment may be adiffusely reflective pigment such as titanium dioxide or lead chromate,or may be a specularly reflective pigment such as pearlescent pigment oraluminum flakes. Alternatively, the microspheres may instead be coatedwith a specular reflector, e.g., dielectric, aluminum, or silver, ontheir embedded portions, hereinafter referred to as coated microspheres.The binder layer may also contain UV-fluorescent pigments. The binderlayer is formed by mixing the pigments into a light-transmissive medium,and then coating or applying this medium onto the integral protrusionsof the base sheet. The important properties for this binder includelight-transmissivity, durability for intended use, ability to keep thepigment particles suspended, and adequate wetting and bead adhesion. Itis important that the coating medium be light-transmissive so that lightentering the retroreflective article is not absorbed by the medium butis instead reflected back. For ease of coating, the medium willpreferably be a liquid with a viscosity of less than 10,000 centipoiseat room temperature. Vinyls, acrylics, epoxies, and urethanes areexamples of suitable mediums. Urethanes such as disclosed in U.S. Pat.No. 4,899,555 (Hedblom) are a preferred medium. Any suitable mixingtechnique that avoids high shear that would deleteriously change theparticles' shape may be used to disperse the pigment in the medium. Thedispersion is then coated onto the integral protrusions of the basesheet. After coating, specular pigment flakes in the designated sizerange and in the designated concentration tend to orient themselves withtheir flat surfaces in approximately tangential relationship with themicrosphere. Preferably, these pigments will bend through their broadestdimension to conform to the contour of the microsphere in a cup-likefashion, thereby providing good retroreflective efficiency.

Specular pigment flakes which may be used in the invention are generallythin, plate-like particles, which are large in comparison to thediffusely reflective pigments commonly used in pavement markings. Thekey property of these pigments is that their length and width is muchgreater than their thickness. Other specular flakes possessing thisproperty may also be used. Due to this property of being much greater inlength than in thickness, these flakes tend to align themselves parallelto the web or surface on which they have been coated. When themicrosphere is dropped on and indents the coating, the flakes tend toline up around the embedded portion of the microsphere like a coating.This property is known as "leafing". This tendency of the flakes toeffectively coat the microsphere results in higher levels ofreflectivity, as the reflectivity will have a larger specular componentthan if this coating did not take place. Particularly preferred specularpigments because of their very large increase in reflectivity arepearlescent pigments and aluminum flakes.

Specular pigments have a large specular reflectance component, so thatlight hitting the pigment particle tends to be reflected at a mirrorimage from normal of the angle at which it entered. These specularlyreflective flakes have an average maximum dimension falling in the rangeof about 4 to about 36 micrometers and an average thickness in the rangeof about 2 to about 5 micrometers, the binder preferably containing atleast about 20 percent by weight based on dry weight of pigment comparedto total weight of medium and pigment. More preferably, the weightpercent of pigment will be higher, in the range of 35 to 40 percent. Ifthe concentration of pigment is too high or too low, the leafing abilityof the pigment may be interfered with, and the brightness decreased. Anexample of a suitable pearlescent pigment is a typical mica-basedpearlescent pigment also containing titanium dioxide available from theMearl Corporation of New York, N.Y. An example of a suitable aluminumflake is ATA 2100, with average particle size of 32 μm, available fromAlcan-Toyo America, Naperville, Ill.

Ultra-violet (UV) fluorescent pigments may also be added to the binderlayer. UV-fluorescent pigments are pigments that fluoresce light in thevisible spectrum when excited by the ultraviolet light from a specialUV-headlamp of a motor vehicle. The UV light from the motor vehicle'sheadlamp falls onto a microsphere, and is diffracted below to fall onthe pigment in the binder layer. The pigment is excited by the incidentUV light, and gives off energy as light in the visible spectrum, some ofwhich is visible to the driver of the motor vehicle. The importantproperty of these pigments are that they are excitable by UV light, andsubsequently emit light in the visible range. These UV-fluorescentpigments will often have a specific daytime color. An example of asuitable UV-fluorescent pigments is Blaze Orange GT-15-N, available fromDayglo Corporation (Nalco Chemical Company) of Cleveland, Ohio. Thefluorescent pigments are located throughout the binder layer.

The transparent microspheres of some embodiments of the invention rangefrom about 25 to about 600 micrometers (μm) in diameter, although largermicrospheres will also work. Preferably, the microspheres range indiameter from 200 to 250 μm. The microspheres of the invention can be ofany material suitable to adequately refract light, such as ceramic orglass. Preferably, the microspheres are ceramic for durability. U.S.Pat. No. 4,564,556 (Lange) teaches the making of ceramic microspheres.The microspheres preferably have an index of refraction greater than1.5. More preferably the index of refraction is greater than 1.7. Forthose embodiments incorporating a diffusely reflective pigment such astitanium dioxide, the index of refraction is preferably about 1.85 toabout 1.90. For those embodiments with specularly reflecting pigments,the index of refraction is preferably about 1.93.

FIG. 4 is a schematic diagram of an embodiment of an uprightretroreflective element of the invention. The light 42 enters themicrosphere 44 which is partially embedded in a binder layer 43comprising specularly reflecting pigments. The incident light 42 isrefracted below to focus at the specularly reflective coating on themicrosphere. The light 46 is reflected at a mirror image from normal ofthe angle at which it entered. For a specularly reflecting element on ahorizontal surface, such as a flat pavement marking, the specularlyreflecting surface returns light at such angles that little or none ofthe incident light is returned to the motor vehicle driver. It is forthis reason that an upright element is critical in order to makeefficient use of the specular optics. It follows then not to lose thisadvantage of a vertical component by covering or shadowing the verticalsides.

Retroreflective articles of the invention may be prepared in thefollowing manner. The binder dispersion is prepared by mixing reflectivepigment flakes and optionally UV-fluorescent pigments in alight-transmissive medium, for example with an air mixer, taking carenot to expose the flakes to high shear so as to maintain theirintegrity. This dispersion is then coated onto the integral protrusionsof a pre-embossed base sheet. The spacing of the protrusions isdetermined as earlier specified. Coating is by any suitable means;squeeze roll coating as disclosed in U.S. Pat. No. 4,988,541 (Hedblom)and U.S. Pat. No. 5,227,221 (Hedblom) is a preferred method.Alternatively, a film of liquid bead bond supported by a release lineror sheet may be laminated to the selected surfaces of the protrusions.

The microspheres are delivered onto the binder layer while it is stillfluid so that the microspheres will sink into the coating and becomepartially embedded to a depth of approximately 50 to 60 percent of theiraverage diameter. This depth of embedment is important for both adequateanchoring and efficient optics. The microspheres are delivered from ahopper usually equipped with a metering blade to control the number ofmicrospheres put on. If the binder is coated from solvent, some of thesolvent may be evaporated prior to placement of the microspheres inorder to improve control of the depth of embedment. Alternatively,excess solvent may be evaporated after microsphere deposition. The depthof embedment, or sink, of the microsphere is controlled through acombination of the relationship between the surface energy of themicrospheres and the surface tension of the coating, and the viscosityof the fluid coating. If the surface energy of the microsphere is closeto the surface tension of the fluid coating, the microsphere will tendto float and not sink entirely into the coating. The surface energy ofthe microsphere can be altered by various surface treatments, such asthe use of surfactants. The viscosity of the coating may be controlled,for example through heating, solvent content, addition of thickeners,selection of composition, or addition of fillers. Thixotropic agents orother thickening agents may be added to the binder in amounts that willnot substantially decrease brightness.

By controlling the surface energy of the microspheres and the rheologyof the coating, the manner in which the binder material draws up aroundthe microspheres is controlled. Capillation is the term used to describethe wicking action of the binder material around the microsphere. Thiscapillation is important because the binder material forms a socket-likestructure around the microsphere and holds it in place. With thecapillation taken into account, the microspheres are embedded on averageto a depth of approximately 50 to 60 percent of their average diameter.As the binder material hardens as by cooling, loss of solvent or othervolatiles, or by chemical reaction such as cross-linking orpolymerizing, the microspheres are held in place.

Alternatively, retroreflective articles of the invention may be preparedby applying retroreflective protrusions to a flat base sheet. A rod ofsuitable material, for example a thermoplastic containing reflectivepigment, is extruded in a desired shape. Retroreflective microspheresmay be applied to the rod which is then sliced into wafers.Alternatively, sheets of retroreflective material may be applied to therod before it is cut into wafers. Particles such as skid-resistantparticles or retroreflective microspheres may be applied to the topsurface of the wafers before or after application to the base sheet ifdesired.

In a particularly preferred embodiment, the base sheet has integralprotrusions similar to those described in U.S. Pat. No. 4,988,555(Hedblom). In commercial versions of the product disclosed herein, theprotrusions described are squares about 1 millimeter (mm) in height andabout 0.63 centimeters (cm) on each side with transparent microspherespartially embedded in and protruding from a binder layer containingreflective pigments. In this particular patent, the squares are in aregularly repeating pattern, and the spacing between the sides of thesquares is about 0.32 cm. The squares are 45° from normal in theobservation axis. Alternating rows are offset or staggered. (see FIG. 1)A particularly preferred embodiment of the invention differs from theembodiment disclosed in that patent in that every second and third rowof protrusions are replaced by flat base sheet, and in that theprotrusions are about 2 to about 3 millimeters (mm) in height. (see FIG.2) Another preferred embodiment of the invention differs from theembodiment disclosed in that patent in that every third through sixthrow of protrusions is removed.

In another particularly preferred embodiment, the protrusions are thesame dimensions with the same spacing as FIG. 2, only turned 45° so thatthe sides of the squares are perpendicular to the observation axis.

EXAMPLES

The invention will be further explained by the following illustrativeexamples which are intended to be non-limiting. Unless otherwiseindicated, all amounts are expressed in parts by weight.

The Coefficient of Retroreflected Luminance (R_(L) in mcd/m² /1×) wasmeasured at typical industry standard geometries, and at geometries moreclosely matching those actually encountered by a motor vehicle driver.Measurements were made according to ASTM D 4061-89. Results are given inthe following table. Intrinsic geometry as described in ASTM E 808-91was used. Entrance/observation angles are stated. Presentation angle waskept constant at 0 degrees, orientation angle was maintained at -180degrees. The entrance/observation angles given are for the leftheadlight, to centerline marking, to observer at each of the distancesspecified.

Example 1

Is a sample of STAMARK™ Brand Pavement Marking Tape Series 380 obtainedfrom the Traffic Control Materials Division of 3M. This materialcontains 225 micrometer ceramic microspheres with an index of 1.75.

Example 2

A piece of Example 1 was modified by leaving two transverse rows ofraised protuberances as is, then removing two rows with a razor blade.The sequence was repeated until a sample large enough for photometricwork was obtained.

Example 3

An example similar to number 2 except that for every two rows thatremained untouched, four rows were physically removed.

Example 4

An example similar to number 2 except that for every two rows thatremained untouched, six rows were physically removed.

Example 5

A second sample of STAMARK™ Brand Pavement Marking Tape Series 380 wasobtained from the Traffic Control Materials Division of 3M.

Example 6

A piece of Example 5 was modified by leaving one transverse row ofraised protuberances as is, then removing two transverse rows with arazor blade. The sequence was repeated until a sample large enough forphotometric work was obtained.

Example 7

A sample of patterned rubber that is used in the product disclosed inExample 1 was squeeze roll coated with a polyurethane resin formulationincorporating 40 percent by weight Fine Pearl pigment (code number 139V,particle size range 4-32 microns, specific gravity 3.2) made by theMearl Corporation of 41 East 42nd Street, New York, N.Y. 10017.Immediately after coating, 1.94 index of refraction ceramic beads wereembedded into the urethane on the sides of the raised protuberances. Theurethane was cured and any excess beads removed. Bead embedment wasdetermined to be near 50 percent.

Example 8

A piece of Example 7 was modified in the same manner as that of Example2.

Example 9

A prototype patterned pavement marking was produced by coating a film ofurethane resin incorporating 33 weight percent titanium dioxide. Ceramicmicrospheres used in Example 5 were embedded into the resin in a closedpacked arrangement, and the urethane then cured. The film was then slitinto strips 0.32 cm in height and 10 cm in length. The strips wereadhered to wooden sticks of the same dimensions, and spaced five inchesapart which closely corresponds to the geometric spacing arrangement ofExample 3 and Example 6.

Example 10

A prototype similar to Example 9 was produced using the pigmentedurethane and ceramic microspheres as described in Example 7.

Example 11

A prototype similar to Example 10 was produced except that thepearlescent pigment was replaced by an equal weight percent of AluminumPigment Flake ATA 2100 (average particle size 32 microns, specificgravity 2.5) made by Alcan-Toyo America, 1717 North Boulevard, Suite201, Naperville, Ill. 60540.

Example 12

A prototype similar to Example 10 was produced except that prior toplacing the ceramic beads into the coating, they were coated with asilver reflecting mirror about their entire surface. Aftersolidification of the coating, the exposed surfaces of the ceramic beadswere then etched with acid, which removes the silver coating from theexposed surfaces of the microspheres. The result was 100 percentoriented, hemispherical silver coated microspheres.

Example 13

A sample of STAMARK™ Brand Pavement Marking Tape Series 380 was cut onthe bias so as to give a row of square, raised reflective elements. Thesquares were parallel to the length of the strip. The strips were thenadhesively attached in parallel transverse rows to a 4 inch wide stripof aluminum sheeting. The spacing between the rows was selected to bethe same as that of Example 6. Very high reflectivity resulted becausethe vertical reflecting surfaces were perpendicular to the direction ofviewing.

    ______________________________________                                        Coefficient of Retroreflected Luminance R.sub.L in                            mcd/m.sup.2 /lx (Left Headlight, Entrance/Observation                         Angle, Presentation = 0, Orientation = -180)                                                   Distance from Left Headlight,                                Conventional     to Centerline                                                Geometries       30 Meters                                                                              50 Meters                                                                              80 Meters                                  Example No.                                                                           86.0/0.2 86.5/1.0                                                                              88.5/1.0                                                                             89.3/0.6                                                                             89.6/0.4                               ______________________________________                                        1       3200     1900    1550   1900   *                                      2       2250     1400    1500   1700   *                                      3       1800      800    1500   2500   *                                      4       1100      500     850   1600   *                                      5       2800     1700    1100   1200   1250                                   6       1700     1050    1300   1500   1700                                   7       28800    2820    1780   1870   2060                                   8       *        *       *      4800   13000                                  9       2370     1670    2470   2370   2330                                   10      36000    2990    5080   9870   14700                                  11      45000    4160    7190   14400  22800                                  12      74800    9330    15900  34100  49400                                  13      *        *       *      *      *                                      ______________________________________                                         * These samples were not measured at this set of photometric angles.     

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention.

What is claimed is:
 1. A pavement marking sheet material comprising acontinuous polymeric base sheet having a plurality of uprightretroreflectors on its upper surface, said upright retroreflectorscomprising retroreflective elements,said upper surface being made up ofa plurality of segments arranged in a selected manner, each said segmenthaving a front edge, a rear edge, and two side edges, and having anarray of a plurality of said upright retroreflectors positioned alongsaid front edge and a spacing zone that is free of said uprightretroreflectors along said rear edge, the longitudinal length L of saidspacing zone being determined by the formula L>H/tan(90°-θ), where H isthe height of the upright retroreflectors and θ is a design entranceangle that falls within the range of 86.9° to 89.9°; said markingmaterial comprising a plurality of segments arranged with the front edgeof a segment abutting the rear edge of an adjacent segment.
 2. The sheetmaterial of claim 1 wherein said segments comprise arrays that comprisea plurality of rows of upright retroreflectors, said rows being parallelto said front edge of said segments.
 3. The sheet material of claim 1wherein said arrays of upright retroreflectors are centered about atleast two distinct longitudinal axes extending through said material. 4.The sheet material of claim 1 comprising a plurality of segmentsarranged with one side edge of a first segment abutting one side edge ofa second segment.
 5. The sheet material of claim 1 wherein at least someof the upright retroreflectors are protuberances formed in said uppersurface of said base sheet, said protuberances having retroreflectiveelements thereon.
 6. The sheet material of claim 1 wherein at least oneretroreflector includes a side surface which forms an angle of at leastabout 60° to the plane of said base sheet.
 7. The sheet material ofclaim 1 wherein said retroreflective elements comprise transparentmicrospheres partially embedded in and protruding from a binder layer.8. The sheet material of claim 1 wherein said pavement marking sheet hasgreater reflectivity than a comparably constructed pavement markingsheet with a greater number of upright retroreflectors.
 9. The sheetmaterial of claim 1 wherein said pavement marking sheet has greaterskid-resistance than a comparably constructed pavement marking sheetwith a greater number of upright retroreflectors.
 10. The sheet materialof claim 1 wherein said pavement marking sheet has increasedconformability to a substrate than a comparably constructed pavementmarking sheet with a greater number of upright retroreflectors.
 11. Thesheet material of claim 1 wherein said retroreflectors comprise cubecorner retroreflective elements.
 12. The sheet material of claim 1 woundupon itself into a roll form.
 13. The sheet material of claim 1 whereinsaid upright retroreflectors have a base shape selected from the groupconsisting of polygon, circle, or oval.
 14. The sheet material of claim13 wherein said upright retroreflectors have a base shape that isrectangular, said upright retroreflectors being oriented on base sheetsuch that one of the diagonals through opposing corners of each elementis substantially parallel with the longitudinal axis of the sheetmaterial.
 15. The sheet material of claim 13 wherein said uprightretroreflectors have a base shape that is rectangular, said uprightretroreflectors being oriented on base sheet such that one of the sidesof each element is substantially parallel with the longitudinal axis ofthe sheet material.
 16. The sheet material of claim 5 wherein saidprotuberances each have at least one dimension in the plane of the basesheet that is less than about 15 millimeters, have a height of at leastabout 1 millimeter, and have a side surface that is adapted to faceoncoming traffic when said sheet material is applied to a roadway andwhich forms an angle to the plane of the base sheet of at least about30°, said side surface carrying retroreflective elements protruding fromthe surface.
 17. The sheet material of claim 5 in which saidprotuberances occupy no more than about fifteen percent of the area ofsaid base sheet.
 18. The sheet material of claim 5 wherein the areasbetween said protuberances form a connected grid of valleys.
 19. Thesheet material of claim 5 in which the areas between protuberances forma connected grid of valleys running diagonally with respect to thedirection of expected road travel.
 20. The sheet material of claim 16wherein all dimensions of said protuberances in the plane of said basesheet are less than about 15 millimeters.
 21. The sheet material ofclaim 16 in which the protuberances have at least one dimension in theplane of said base sheet that is less than about 10 millimeters.
 22. Thesheet material of claim 16 wherein all dimensions of said protuberancesin the plane of said base sheet are less than about 10 millimeters. 23.The sheet material of claim 7 wherein said microspheres have an index ofrefraction of between about 1.75 and about 2.0.
 24. The sheet materialof claim 7 wherein said microspheres have an average diameter from about50 micrometers to about 600 micrometers.
 25. The sheet material of claim7 wherein said microspheres are embedded to a depth of approximately 40to 65 percent of their average diameter.
 26. The sheet material of claim7 wherein said binder layer comprises ultraviolet-fluorescent pigments.27. The sheet material of claim 7 wherein said binder layer comprisesflakes of a reflective pigment.
 28. The sheet material of claim 27wherein said binder layer comprises flakes selected from the groupconsisting of aluminum particles and nacreous pigments.
 29. The sheetmaterial of claim 27 wherein said binder layer comprises a diffuselyreflective pigment.
 30. The sheet material of claim 28 wherein saidflakes comprise 10 to 50 percent by weight of the total weight of thepigment flakes and binder medium combined.
 31. The sheet material ofclaim 28 wherein said flakes comprise 30 to 45 percent by weight of thetotal weight of the pigment flakes and binder medium combined.
 32. Thesheet material of claim 28 wherein said flakes range in average lengthfrom about 4 micrometers to about 36 micrometers.
 33. The sheet materialof claim 28 wherein said flakes tend to orient themselves with theirflat surfaces in approximately tangential relationship with themicrosphere near the bottom of the microsphere, bending through theirbroadest dimension to conform to the contour of the microsphere in acup-like fashion.
 34. The sheet material of claim 28 wherein saidflakes, while bending through their broadest dimension to conform to thecontour of the microsphere in a cup-like fashion, are a greater distancefrom the microsphere near the midpoint of the microsphere where thecontact with the binder ends than at the bottom of the microsphere. 35.A pavement marking sheet material comprising a substrate having a basesurface and a structured surface opposite said base surface, saidstructured surface including:a first retroreflection zone including atleast one .retroreflector; a spacing zone adjacent said firstretroreflection zone, said spacing zone having a length, L, extendingalong a longitudinal axis of said material determined by the formulaL>H/tan(90°-θ), where H is the height of the upright retroreflectors andθ is a design entrance angle that falls within the range of 86.9° to89.9°; and a second retroreflection zone adjacent said spacing zone,said second retroreflection zone comprising at least one retroreflector.36. The pavement marking material of claim 35, wherein:said spacing zoneis completely free of retroreflective elements.
 37. A pavement markingsheet material comprising a substrate having a base surface and astructured surface opposite said base surface, said structured surfaceincluding a plurality of retroreflection zones separated by spacingzones, wherein:each retroreflection zone includes a plurality ofretroreflectors arranged to form an array of adjacent retroreflectors;and each spacing zone has a length L extending along a longitudinal axisof said material determined by the formula L>H/tan(90°-θ), where H isthe height of the upright retroreflectors and θ is a design entranceangle that fails within the range of 86.9° to 89.9°, said spacing zonebeing substantially free of retroreflective elements.
 38. The sheetmaterial of claim 1, wherein:the design entrance angle 0 falls withinthe range of 88.5° to 89.9°.
 39. The sheet material of claim 1,wherein:the design entrance angle θ falls within the range of 89.3° to89.9°.
 40. The sheet material of claim 35, wherein:the design entranceangle θ falls within the range of 88.5° to 89.9°.
 41. The sheet materialof claim 35, wherein:the design entrance angle θ falls within the rangeof 89.3° to 89.9°.
 42. The sheet material of claim 37, wherein:thedesign entrance angle θ falls within the range of 88.5° to 89.9°. 43.The sheet material of claim 37, wherein:the design entrance angle θfalls within the range of 89.3° to 89.9°.